1. Field of the Invention
This invention relates to a method for controlling heat of a thermal head in a thermal printer for carrying out a printing on a recording medium such as form, film or the like, and is particularly concerned with a method for controlling heat of a thermal head for which a heat generated state is properly controlled to produce a better quality printing.
2. Description of the Prior Art
Generally, a thermal head is formed to have a plurality of heat-generating elements arrayed in a single or plural row on a metallic or ceramic substrate, and a printing operation is performed by actuating each heat-generating element by applying a current selectively thereto according to printing information.
The thermal head brings about a temperature rise on each heat-generating element and substrate at the time of printing, therefore in consideration of the temperature rise, an arrangement is such that a power-on time to each heat-generating element is adjusted to prevent an uneven printing from resulting.
For the correction, a past record correcting method and an area correcting method were employed.
The past record correcting method comprises correcting a power-on time in accordance with a past power-on state of the heat-generating element to which to carry a current and its neighboring heat-generating elements.
On the other hand, temperature of the substrate gradually rises from carrying a current to the heat-generating element, therefore if correction is not applied there may arise a difference in print density between the beginning and the end of a line, and hence the power-on time will be corrected therefor in the area correcting method.
A power-on time T.sub.on for the area correction is controlled by a drive circuit wherein, as shown in FIG. 2, a control command is written by turns in a plurality of latches connected to a data bus, thereby outputting a power to each heat-generating element. For obtaining output to 48 dots (48 heat-generating elements) by means, for example, of an 8-bit latch, six latches will be used. Further, a control of the power-on time by the drive circuit is effected according to the control command written in each latch from accessing by turns to the plurality of latches through the data bus, therefore the power-on time to each heat-generating element cannot be controlled by a resolution finer than a minimum power-on time T.sub.res (resolution) expressed by the product TN of a time T for accessing one latch and a number N of the latches, and hence the power-on time to each heat-generating element is controlled by means of a value given integral times as high as the minimum power-on time (T.sub.res .times.n).
However, in the prior art controlling method for heat of a thermal head described as above, since a power-on time to each heat-generating element is controlled by means of the minimum power-on time multiplied integrally, a problem inherent therein is such that a fine control of the power-on time for securing a quality print is no more realizable for the recent high-speed requirement of printing, a multiplicity of the heat-generating elements (characters more than 48 dots being used generally on the latest thermal printer) and so forth.
Additionally, a size of each heat-generating element is 0.1 mm or below, and hence when the power-on time to each heat-generating element is to be controlled minutely, a drive circuit complicate in construction must be used, a big-sized system is entailed all the more and a cost increases inevitably as well.